The need for higher bandwidth efficiency for emerging networking technologies and the fact that today's licensed bandwidth is inefficiently utilized has lead to the development of technologies in cognitive radios (CR). A CR-enabled device is able to communicate on bands licensed to other devices by performing a spectrum sensing operation to find available spectrum at a particular instant and using the available spectrum at times where it may not be occupied by the primary user. In order to ensure that CR networks may coexist with current licensed technologies, CR devices may require a high degree of agility, (ability to quickly detect the presence of a primary user and react to the arrival of a primary user), in order to sense the arrival of a primary user on a band and to move to another available band without causing interference to the primary user.
Cooperative spectrum sensing may be used to decrease the sensitivity requirements of the spectrum sensing algorithm for a CR node which may be in a deep fading environment. In the most traditional form of cooperative spectrum sensing, a set of CR nodes may perform spectrum sensing simultaneously and transmit the individual results to a central node to determine the spectrum availability. This requires a method for coordinating and exchanging the spectrum sensing between users.
The simplest form of spectrum sensing algorithm may be the category of energy detection. This category of algorithms detects the presence or absence of a primary user by a measure of the energy detected on the particular band of interest. The simplicity of this form of spectrum sensing makes it highly attractive for the CR application. For example, the IEEE 802.22 standard for wireless regional area networks (WRANs) made energy detection using white spaces in the TV spectrum one of the accepted spectrum sensing techniques. A major challenge using energy detection for the CR device is to distinguish between a primary user transmission and another CR device transmission when performing the spectrum sensing. In addition, for cost reasons, a CR device may contain only one receiver and may not be able to perform spectrum sensing simultaneously with normal transmit (TX) receive (RX) operations. The resulting scenario where many CR devices form an ad-hoc or CR network and may coexist, spectrum sensing periods for each of these nodes need to be coordinated in time to ensure that the spectrum sensing is performed when no other CR nodes in the vicinity are transmitting. This leads to what is called a silent period for spectrum sensing.
The use of silent measurement periods in a network was explored for wireless access points (AP) needing to perform measurements on an allowable channel set. This previous work does not address the needs of a CR network trying to perform spectrum sensing in an environment with different primary users using differing wireless technologies. For example, the silent measurement period (SMP) requested by an AP may only be initiated when the communication medium becomes available, which may not address the agility requirements of a CR device. The CR device may react to the arrival of a primary user by switching to an unused band in a limited delay not to incur any interference to the primary user.
For example, the IEEE 802.22 draft standard requires an evacuation time of two seconds. This is the time required for a secondary user to leave the currently used band following the arrival of a licensed primary user to that band. Typically, periodic spectrum sensing periods have been considered, which require the CR nodes to perform spectrum sensing at least every two seconds to be able to achieve the required evacuation time. In general, periodic spectrum sensing may result in higher overhead in the CR node due to context switching, maintaining of buffers and real-time traffic during the spectrum sensing times. As a result, short spectrum sensing periods are highly undesirable from an efficiency perspective. Accordingly a method for coordinating silent periods for spectrum sensing in a CR network is needed.